1. Technical Field
This invention relates to electronic imaging devices, and more particularly to metal-oxide-semiconductor imaging devices utilizing a photoelectric conversion layer.
2. Background Information
A number of technologies have been used for solid state imaging devices, including charged coupled device (CCD) arrays and photoconductor on thin film transistor arrays. More recently, use of metal-oxide-semiconductor (MOS) arrays (particularly complimentary metal-oxide-semiconductor (CMOS) arrays) have been used in conjunction with a photoelectric conversion layer to provide a random-access imaging device having a number of beneficial qualities. MOS fabrication technology is a well established industry which involves fabrication of integrated circuits on and in the upper surface of a wafer of crystalline silicon. Complimentary metal oxide semiconductor (CMOS) technology combines both n-channel and p-channel transistors on a single wafer. MOS technology typically utilizes a single crystal silicon substrate as the semiconductor material for transistor fabrication. Use of MOS technology allows image producing electronics and image processing electronics to be integrated, a configuration sometimes called active pixel sensors (APS). Further background information concerning such devices may be found in U.S. Pat. No. 5,528,043, entitled "X-ray Image Sensor", and assigned to the assignee of the present invention, the contents of which are hereby incorporated by reference.
FIG. 1 is a stylized diagram of a prior art MOS imaging array, showing the structure for one picture element (pixel). Such structures are conventionally fabricated on a silicon substrate 10, on or in which MOS or CMOS circuitry (including interconnects) 12 of various types is fabricated using conventional techniques. Distinct pixel circuit collection nodes 14 each defining a picture element are included as part of the circuitry 12. As is known in the art, each pixel collection node 14 is in contact with a conductive pixel pad 16, which is typically aluminum or an aluminum alloy. Each pixel pad 16 (only one of an array of such pads 16 is shown) is separated by a layer of insulating material 18, which may be, for example, SiO.sub.2 or Si.sub.3 N.sub.4. Additional deposits of an insulating material 19, which may be, for example, SiO.sub.2 or Si.sub.3 N.sub.4, isolate adjacent pixel pads 16.
In a conventional design, a photoelectric conversion layer 20 of intrinsic material is overlaid on top of the pixel pads 16. An optional n-type or p-type doped semiconductor top layer 22 may be formed over the photoelectric conversion layer 20 to create a diode structure. An electrode layer 24 is then formed on top of the semiconductor top layer 22 if present, or on top of the photoelectric conversion layer 20. For most applications, the electrode layer 24 is formed of a thin transparent conductive oxide (TCO) material such as indium tin oxide.
In operation, an electric field is generally created between the electrode layer 24 and the pixel pads 16. Photons 26 passing through the electrode layer 24 interact with the photoelectric conversion layer 20 and generate electron-hole pairs. Because of the applied electric field, each hole or electron (depending on the field direction) is drawn towards a nearby pixel pad 16 before recombination occurs. Differences between the amount of charge induced on the pixel pads 16 create differences in the charge collected on the pixel collection nodes 14. The collected charge can be sensed and read out as an image, in known fashion.
A problem with the configuration shown in FIG. 1 is that the materials that can be used for the photoelectric conversion layer 20 are limited to those materials that do not adversely interact with the material of the pixel pads 16. The pixel pads 16 are typically made principally of aluminum. Aluminum often does not form a good diode junction with intrinsic semiconductor material. Further, the material comprising the photoelectric conversion layer 20 and the process for forming or depositing the photoelectric conversion layer 20 must be compatible with aluminum, which restricts availability of desirable materials. Further, some materials otherwise useful for forming the photoelectric conversion layer 20 may exhibit electromigration during operation if in contact with aluminum. Such interaction or migration can adversely effect the performance of the device, and ultimately can cause failure of the device.
Another problem with the configuration shown in FIG. 1 is that the top-most electrode layer 24 can block certain desirable wavelengths. In particular, it is difficult to find a material for the electrode layer 24 that does not block ultraviolet (UV) wavelengths.
The inventors have determined that it would be useful in a MOS or CMOS imaging array to be able to avoid using aluminum pixel pads in some embodiments. The inventors also have determined that it would be useful in a MOS or CMOS imaging array to be able to avoid using a top-most electrode layer in some embodiments. The present invention provides methods and structures for accomplishing these objects.